Wnt proteins are secreted glycoproteins acting as growth factors that regulate various cellular functions include proliferation, differentiation, death, migration, and polarity, by activating multiple intracellular signaling cascades, including the β-catenin-dependent and -independent pathways. There are 19 Wnt members have been found in humans and mice, and they exhibit unique expression patterns and distinct functions during development. In humans and mice, the 10 members of the Frizzled (Fz) family comprise a series of seven-pass transmembrane receptors that have been identified as Wnt receptors. In addition to Fz proteins, single-pass transmembrane proteins, such as low-density lipoprotein receptor-related protein 5 (LRP5), LRP6, receptor tyrosine kinase (RTK)-like orphan receptor 1 (Ror1), Ror2, and receptor-like tyrosine kinase (Ryk), have been shown to function as co-receptors for Wnt signaling. Therefore, it has been assumed traditionally that the binding of different Wnts to their specific receptors selectively triggers different outcomes via distinct intracellular pathways.
Diverse Wnts, Wnt receptors, and downstream pathway all contribute to the role of Wnt. These pathways all play a role in development, stem cell maintenance, cancer and metastasis.
In the absence of Wnt signaling, β-catenin is bound and phosphorylated by a “destruction complex” containing the adenomatous polyposis coli (APC) and Axin proteins, as well as glycogen synthase kinase 3 (GSK3) and casein kinase I (CKI). Phosphorylated β-catenin is bound by the F box protein Slimb/β-TrCP and polyubiquitinated, leading to proteosomal degradation. In addition, the complex acts to prevent nuclear localization of b-catenin. Upon Wnt binding to Frizzled (Fz) and low-density lipoprotein-related proteins 5 and 6 (LRP5/6), GSK3, Axin, and other destruction complex components are recruited to the receptor complex. The function of the destruction complex is inhibited, and unphosphorylated β-catenin accumulates in the cytoplasm and eventually translocates to the nucleus. There, it associates with TCF proteins, converting TCF from a repressor into an activator of Wnt-responsive gene transcription.
Wnt in Cancer & Stem Cell:
Deregulation of components of Wnt/β-catenin signaling is implied in a wide spectrum of diseases including degenerative diseases, metabolic diseases, and a number of cancers such as cervical, colon, breast, bladder, head and neck, gastric, lung, ovarian, prostate, thyroid, non-small-cell lung, as well as chronic lymphocytic leukemia, mesothelioma, melanoma, pancreatic adenocarcinoma, basal cell carcinoma, osteosarcoma, hepatocellular carcinoma, Wilm's tumor and medulloblastoma.
Wnt signaling plays a role both during development, and within stem cell niches in adults. This is best established in skin, hematopoietic stem cells, mammary gland and in intestinal proliferation. For example, high level expression of DKK1, an inhibitor of Wnt signaling, blocks normal stem cell proliferation in mouse intestine, suggesting there is an essential role for Wnt signaling in maintenance of stem cells in the digestive tract. Wnt roles in self renewal and expansion of stem cells have also been demonstrated for embryonic and neural stem cells, suggesting that Wnt signaling may be a general requirement of stem cell maintenance. Inhibition of Wnt signaling, e.g., by overexpression of axin or an extracellular Wnt-binding protein, sFRP, reduces hematopoietic stem cell (HSC) growth in vitro and the ability to reconstitute HSCs in vivo. Notably, while overexpression of activated β-catenin can expand HSC populations in culture for extended periods, two groups have reported that β-catenin is not required for HSC survival and serial transplantation, supporting the proposal that there is more to Wnt signaling than stabilization of β-catenin in stem cell survival. Diverse Wnts can regulate stem cell proliferation: Wnts 1, 5a, and 10b are able to stimulate expansion of HSC populations and Wnt5a acts synergistically with stem cell factor (SCF) to expand and promote self renewal of HSCs. The demonstration of a role for Wnt5a in HSC self renewal and its ability to synergize with stem cell factor is particularly interesting because Wnt5a often acts in a β-catenin independent manner. While Wnt signaling is critical for stem cell maintenance, it may therefore be via signaling pathways distinct from or in parallel to the β-catenin pathway.
Fibrosis:
Wnt/β-catenin signaling pathway is essential to embryonic development in general and organ morphogenesis, so it is not surprising that dysregulation of this pathway in adult has been linked to fibroblast biology and fibrosis. It has been demonstrated that Wnt/β-catenin signaling play a role in severe fibrotic diseases, such as pulmonary fibrosis, liver fibrosis, skin fibrosis and renal fibrosis.
Others:
Dysregulation of Wnt/β-catenin signaling contributes to the development of diabetic retinopathy by inducing retinal inflammation, vascular leakage, and neovascularization.
The binding of Wnt proteins to plasma membrane receptors on mesenchymal cells induces the differentiation of these cells into the osteoblast lineage and thereby supports bone formation. Wnts are also key signaling proteins in joint remodeling processes. Active Wnt signaling contributes to osteophyte formation and might have an essential role in the anabolic pattern of joint remodeling that is observed in ankylosing spondylitis and osteoarthritis. By contrast, blockade of Wnt signaling facilitates bone erosion and contributes to catabolic joint remodeling, a process that is observed in rheumatoid arthritis.
There is therefore a need for compounds that modulate and/or inhibit the WNT pathway so as to treat diseases associated with WNT activity.